This invention is directed to a method of recovering thermal or heat energy of a subterranean coal formation by burning the coal in situ and transporting the generated heat to the earth's surface.
Coal has long been used as a fuel and represents a large percent of the proven reserves of all of the earth's fossil fuels. Coal is burned directly as a fuel and has also been gasified to produce flammable gas. Gasification of coal has often been carried out at the earth's surface. This requires that coal be mined and transported to the gasification site. Techniques have also been taught for burning and gasification of coal in situ to produce flammable gases and coal tar liquids.
In U.S. Pat. No. 3,628,929 to Eugene D. Glass and Vaughan W. Rhoades there is described a method for the production of flammable gas and coal tar liquids by the in situ combustion of a coal bed. The process consists of completing wells in the coal seam, creating a horizontal fracture in the coal seam to establish communication in the wells, igniting the coal about the injection well, injecting a combustion-supporting gas, and recovering volatile liquids at the production well. The injection wells are cased and cemented to the center of the coal layer to enhance the fracturing operation. The formation of a horizontal fracture within the coal bed is an essential criterion of the process for success of the entire operation. The fracture is initiated as close to the center of the coal bed within the injection well as feasible and propagated through the bed to intersect the surrounding production wells. A hydraulic fracturing technique is discussed for creating the horizontal fracture. It is pointed out that though it is common practice to separate a fractured network by the use of propping agents, that in the combustion of a coal seam propping agents should not be used. Propping agents would tend to coagulate the coal tar products produced in the production of a coal seam and block the fracture. A preferred method is described whereby water is simultaneously injected with the combustion-supporting gas to control the temperature of the flame front and to adjust the calorific value of the produced gas. A water-gas shift reaction takes place at the site of the combustion front and yields a considerably enhanced calorific content produced gas.
In U.S. Pat. No. 3,775,073 to Vaughan W. Rhoades there is described another technique for the in situ combustion of a coal seam to recover flammable gases and coal tar liquids. Wells are completed in a coal seam and a horizontal fracture is created in the coal seam to establish communication between the wells. A first combustion-supporting gas is ignited within the horizontal fracture so as to form a horizontal and vertical fractured network between the wells. A second combustion-supporting gas is injected to propagate the combustion front through the coal about the injection well and the volatile liquids and combustible gases are recovered at the production well. The formation of a horizontal fracture within the coal bed is considered to be an essential criterion of the invention for success of the entire operation. It is said that the fracture should be initiated as close to the center of the coal bed, within the injection well, as feasible and propagated through the bed of the coal seam so as to intersect the surrounding producing wells.
In U.S. Pat. No. 3,138,203 to Malcolm A. Weiss et al. there is described an in situ combustion method of recovering oil from an underground oil-bearing stratum wherein a burning front is caused to channel toward one or more producing wells, preferentially along the lower boundary of the stratum. An underground burning operation is conducted so as to deliberately cause the combustion front to travel selectively from the input wells to the output wells along the bottom of one or more oil-bearing strata within the reservoir. Techniques described for propagating a combustion front along the lower boundary include opening the input and output wells solely at the bottom of each stratum of interest as by perforating, fracturing, or the like. The fractures may be extended part or all of the way between the input and output wells.
In U.S. Pat. No. 3,520,363 to Charles L. Bauer there is described a process for the recovery of hydrocarbons from subterranean hydrocarbon-bearing formations by conducting an in situ combustion operation prior to water injection. In situ combustion is caused to occur in random pockets in a subterranean formation followed by water injection thereinto to scavenge the generated heat as part of a water drive and thereby produce additional hydrocarbons contained therein.
Techniques wherein subterranean formations are hydraulically fractured to improve the recovery of hydrocarbons from hydrocarbon-bearing formations have been widely used. Further, techniques wherein oil shale formations and coal-bearing formations are hydraulically fractured to improve the recovery of hydrocarbons from these formations have been described.
In U.S. Pat. No. 3,285,335 to Carl E. Reistle, Jr. there is described a method of in situ pyrolysis or recovery of organic carbonaceous materials from subterranean deposits of oil shale. A number of substantially horizontal shafts are drilled though an oil shale formation such that at least a pair of the shafts are in different horizontal planes and generally oriented in the same direction. Fracturing operations are conducted from at least one of the shafts so that the plane of the resulting fracture is vertically disposed so as to open communication between at least a pair of horizontal shafts. A thermal fluid having a temperature sufficient to pyrolyze the oil shale is introduced into the formation along the parted fractured plane whereby the sensible heat of the fluid is transferred to the surrounding oil shale formation and pyrolysis and vaporization of the kerogen content of the oil shale occur. The products of pyrolysis and vaporization are entrained in the thermal fluid and are recovered therewith at the earth's surface. In accordance with another aspect, the preferred fracture orientation in the oil shale formation is determined and the work shaft is drilled into the oil shale formation and from the work shaft a plurality of substantially horizontal shafts are drilled into the formation in order to penetrate the formation at angles not less than 30.degree. and as near as 90.degree. as possible relative to the preferred fracture orientation of the oil shale. The formation is then fractured from at least one of the horizontal shafts to open communication with at least one other of the horizontal shafts and thermal fluid is circulated to pyrolyze the oil shale and conduct the products of pyrolysis to the earth's surface.
In U.S. Pat. No. 3,835,928 to Malcolm K. Strubhar and Edwin E. Glenn, Jr. there is described a method of forming from a deviated well a plurality of vertically disposed fractures spaced a substantial distance apart in a subterranean formation having a known preferred fracture orientation.
In U.S. Pat. No. 3,863,709 to John L. Fitch there is described a method and system for recovering geothermal energy from a subterranean geothermal formation having a preferred vertical fracture orientation wherein at least two deviated wells are provided which extend into the geothermal formation in a direction transversely of the preferred fracture orientation and a plurality of vertical fractures are hydraulically formed to intersect the deviated wells. A fluid is injected via one well into the fractures to absorb heat from the geothermal formation and the heated fluid is recovered from the formation via another well.
In U.S. Pat. No. 3,878,884 to Cecil B. Raleigh there is described a method for producing multiple fractures in earth formations in which the lines of least principal stress deviate substantially from the vertical. It is said that the fracturing method can be used to increase the recovery of gas or oil from low permeability formations and can also be used for in situ recovery of oil from oil shale, or solution mining, or extracting geothermal energy from subterranean formations.